In industry, the use of metal products manufactured by compacting and heat-treating soft magnetic powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced, and different quality requirements are placed on these products depending on their final use. In order to meet the different requirements the powder metallurgy industry has developed a wide variety of iron and iron-based powder compositions.
One processing technique for producing the parts from these powder compositions is to charge the powder composition into a die cavity and compact the composition under high pressure. The resultant green part is then removed from the die cavity and heat-treated. To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubrication is generally accomplished by blending a solid, particular lubricant powder with the iron-based powder (internal lubrication) or by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication). In some cases, both lubrication techniques are utilized.
Lubrication by means of blending a solid lubricant into the iron-based powder composition is widely used and new solid lubricants are developed continuously. These solid lubricants generally have a density of about 1-2 g/cm3, which is very low in comparison with the density of the iron-based powder, which is about 7-8 g/cm3. Additionally, in practice the solid lubricants have to be used in amounts of at least 0.6% by weight of the powder composition. As a consequence the inclusion of these less dense lubricants in the composition lowers the green density of the compacted part.
In modern PM technology, lubrication with only liquid lubricants has not been successful due to poor powder properties and handling. However, liquid lubricants have been suggested for use in combination with solid lubricants. Thus, the U.S. Pat. No. 6,537,389 discloses a method of manufacturing a soft magnetic composite material. In this method punching oil or rapeseed oil methyl ester are mentioned as examples of suitable lubricating additions in the powder composition to be compacted. These compounds are suggested to be used in combination with the solid stearic acid amide lubricant but nothing is taught about the physical nature of the punching oil or rapeseed oil methyl ester and no actual examples demonstrate the use of these compounds. The use of liquid lubricants is also known from U.S. Pat. No. 3,728,110 which teaches that the liquid lubricant should be used in combination with a porous silica gel. Also in this case the liquid lubricant should be combined with a solid lubricant.
It has now unexpectedly been found that when soft magnetic iron or iron-based powders of a certain type are combined with a specific type of liquid organic substances as lubricants, it will be possible to obtain compacted bodies having not only high density but it has also been found that these compacted bodies can be ejected from the dies with comparatively low ejection forces. Furthermore, it has turned out that these lubricants are effective in preventing wearing of the walls of the die and providing the compacted bodies an excellent surface finish. No silica gel is needed for the lubrication.